Obesity is the result of impairment in energy and macronutrient balance. That is, a prolonged positive energy balance (intake > expenditure) and decreased ability to utilize fat (intake > oxidation), contributes to deleterious alterations in body composition observed with aging. It is presently unknown however, whether advancing age is associated with decreased energy expenditure and fat oxidation in response to environmental stimuli (e.g. exercise training). A reduced capacity to increase energy expenditure and fat oxidation would induce an accelerated accumulation of body fat. Increased body fatness is directly linked to the pathogenesis of diseases, including diabetes mellitus, coronary artery disease, and stroke. Our overall hypothesis is that older individuals will show a smaller increase in fat oxidation and total daily energy expenditure than younger individuals in response to endurance training. To directly test this hypothesis, we will: 1) examine the effects of moderate intensity endurance training on fat oxidation in younger and older women, and 2) on total daily energy expenditure and its components (resting metabolic rate and physical activity energy expenditure) in younger and older women using stable isotope methodologies. We will recruit 56 older (50-70 yr) and 28 younger (18-35 yr) women for the study. Volunteers will undergo six months of progressive endurance training (bike ergometer), based on the HRmax. obtained during a VO2max test. Energetic expenditure during each session will be matched in younger and older individuals. Free-living daily energy expenditure and its components (resting metabolic rate and physical activity energy expenditure) will be determined from doubly labeled water and indirect calorimetry. In-vivo fat oxidation will be measured from infusions of 13C-palmitate. The results from this study will provide an insight into energetic and substrate adaptations to exercise (i.e., decreased energy expenditure and fat oxidation), that may explain age-related alterations in body composition (e.g. increased body fatness).